The overall objective of this research proposal is to study carbohydrate tolerance by measuring the pulmonary excretion rate of hydrogen gas (VEH2) as an index of the functional capacity of the intestine to effectively utilize breast milk and commercially prepared infant formulae in which the disaccharide, lactose, is the major sugar. Our ability to estimate the functional capacity of the intestine to effectively utilize the disaccharides is dependent upon the fact that hydrogen gas (H2) is produced in the gastrointestinal tract by certain bacteria from nonabsorbed sugars, and is exhaled in an amount proportional to intestinal H2 production, reflecting the degree of sugar malabsorption. Studies involving human infants will be conducted in the nurseries of Stanford University Medical Center, and will employ exclusively our noninvasive technique for determining VEH2 to establish normal ranges of rate for infants of varying gestational and postnatal ages on standard feeding regimens, and to identify significant variance from the normal ranges occurring among infants during the neonatal and infancy periods.